The present invention relates to a functional device having a functional layer, such as a thin film transistor, a dielectric capacitor, or a solar battery, and a method of manufacturing the same.
Since the pn junction of a hydrogenated amorphous silicon was developed in 1976, the hydrogenated amorphous silicon has been being actively studied. The hydrogenated amorphous silicon has a structure in which a dangling bond in a network made of silicon is terminated by hydrogen or fluorine, and its film can be formed at a low temperature equal to or lower than 300° C. Consequently, the film can be formed on a cheap glass substrate. A study is being made to apply the hydrogenated amorphous silicon to a functional device such as a thin film transistor (TFT), a solar battery, or an optical sensor.
However, when the hydrogenated amorphous silicon is used as it is, in the case of a TFT, only carrier mobility as low as about 0.1 to 0.5 cm2/V·s can be obtained. In the case of a solar battery, there are drawbacks such that doping efficiency is lower as compared with the case of using polysilicon, and photoelectric conversion efficiency deteriorates due to an increase in series resistance. In recent years, a method of solving the problems by irradiating amorphous silicon formed on a glass substrate with an energy beam such as exicimer laser beam so as to be crystallized is being studied. Recently, crystallization of not only semiconductors but also oxides performed by irradiation of an energy beam is also being studied.
In the functional devices, a substrate for supporting a functional layer made of silicon, oxide, or the like is required to be light, shock-resistant, and flexible so as not to be broken when some stress is applied. Conventionally, a silicon substrate, a glass substrate, or the like is used. Recently, it is proposed to use a substrate made of an organic material such as polyethylene terephthalate (PET) which is lighter and more shock-resistant (refer to Japanese Unexamined Patent Application Nos. 8-186267, 10-144930, and 10-144931).
An organic material substrate has, however, a thermal expansion coefficient higher than that of a glass substrate or a silicon substrate. For example, as shown in FIG. 9, when a functional layer 103 is crystallized by irradiating a laser beam LB as an energy beam, problems arises such that a substrate 101 expands by a heat transmitted via an inorganic heat resistant layer 102 to the substrate 101, a very large stress instantaneously works on the functional layer 103, a crack occurs and, in a worse case, peeling occurs. In this case, when the inorganic heat resistant layer 102 for suppressing thermal conduction from the functional layer 103 is formed with a thickness of 500 nm or more, expansion of the substrate 101 is suppressed and peeling of the functional layer 103 can be suppressed to a certain extent. However, even small deformation of the substrate 101 causes a crack in the inorganic heat resistant layer 102 on the substrate 101, and peeling occurs from the interface. In the case of manufacturing a functional device by using the organic material substrate, therefore, sufficient characteristics and reliability cannot be obtained.
The invention has been achieved in consideration of the problems and its object is to provide a functional device having no crack and capable of delivering good functional characteristics and a method of manufacturing the same.